German patent documents DE-A 35 34 045 and DE-A 37 32 641 and International patent document WO-A 85/00790 describe aircraft towing tractors of this type which have a device for engaging and locking the nose wheel undercarriage of an aircraft to the tractor itself. The invention preferably is intended to apply to such aircraft-towing structures. The invention can, however, also apply to tractors connected via a tow bar with the undercarriage of an aircraft nose wheel as is described, for example, in U.S. Pat. No. 4,113,041.
With aircraft tow tractors there is a significant problem in that the loading of the nosewheel undercarriage should not, upon towing, exceed the stresses and loading which are applied thereto during takeoff, landing and the rolling of the aircraft on the runways.
In a towing mode, however, upon acceleration from standstill and upon braking of the towing tractor, forces may be applied to the nosewheel undercarriage which can exceed these stresses.
As a consequence, U.S. Pat. No. 4,113,041 continuously measures the forces in the tow bar and controls the drive engine and/or the braking of the tractor so that the force in the tow bar is maintained within a certain limit.
In International patent document WO A 85/00790 it has been proposed to use such a system also in a tractor with direct engagement by the tractor of the nosewheel undercarriage, i.e. a system in which a tow bar is not used. This known system with direct-force measurement and force limitation, which can also be used if desired with the system of the present invention, cannot fully solve all of the problems encountered heretofore with the towing of an aircraft in the manner described.
For example, with respect to the possibility that the maximum tolerable loading of the nosewheel undercarriage can be exceeded in the tow operation, it should be noted that reliable limitation of the force applied to the undercarriage cannot be assured if only because the limiting force cannot be accurately calculated or determined. For example, for reliability and long-term useful life, the loading and nosewheel undercarriage must remain well below certain limits at which noticeable destruction can commence.
In other words, even with stresses well below the maximum permissible load, repeated applications of the stress or alternating stress and stress relief can result in deterioration because of material fatigue.
Aircraft manufacturers define a further load limitation which is substantially lower than the maximum permissible load and can be approximately one-quarter thereof which has been designated as the fatigue load and have even mandated how often in operation this fatigue load can be achieved.
In other words, the nosewheel undercarriage is so constructed that a predetermined maximum number of force peaks reaching the level of the fatigue load must be excluded to avoid the danger of fatigue rupture. These factors must be taken into consideration in addition to peak loads. After the predetermined number of fatigue load stresses have been applied, the nosewheel carriage must be inspected for fatigue failures.
The number of fatigue load stresses applied in the takeoff and landing procedures are known from the number of flights and these can be recognized for a particular aircraft without difficulty. However, when the aircraft is towed by a tractor, the number of times the fatigue maximum is exceeded can be increased considerably because of large numbers of starting ups and brakings of the tractor.
With conventional tractors for the towing of aircraft, no means is provided which can determine the loading of the nosewheel undercarriage below the maximum permissible loading or which can be used to control or limit the undercarriage loading in response thereto.
Furthermore, the tractor must be capable of towing aircraft of various types and with various load tolerances at the nosewheel undercarriage. The mass to be towed can vary widely. Even when the tractor is designed to tow only large aircraft, the mass to be towed can vary, for example, in the range of 100 to 400 metric tons. The maximum permissible tensile stress to be applied to the nosewheel undercarriage in the longitudinal direction can vary in accordance with the aircraft type between 15 and 52 metric tons. Even when the conventional systems for limiting the maximum load, there is a problem in so limiting the tensile stress applied by the towing vehicle that it should not exceed a load limit of 15 tons, for example, in the towing of the largest aircraft of total weights of 400 tons for which the towing force of 15 tons may be insufficient.
In the prior art system, it has been proposed to eliminate this drawback by providing a controller or regulator for the towing vehicle to which, before each towing operation, data as to the type and the weight of the aircraft towed are introduced by hand. This proposal has been found to be less than fully satisfactory because it is highly prone to errors and defective entry, leading to the potential for serious danger.
This problem is enhanced when the permissible fatigue load of the nosewheel undercarriage must be taken into consideration. In typical conditions, this can lie between 4 and 13 tons. A towing tractor whose towing force must be limited in accordance with the aforedescribed principles that, for example, the smallest permissible fatigue load of 4 tons, cannot be exceeded, is seldom capable of moving the vehicle from one place to another.
It should be apparent, therefore, that earlier systems which take into consideration the loading applied in the towing operation to the nosewheel undercarriage may introduce problems with respect to towing generally. Systems which do not take into consideration the fact that aircraft of different types require different towing forces and cannot adequately deal with the fatigue load problem are unsatisfactory.